This invention relates to the production of finely divided refractory hard metals, that is, borides, carbides, and nitrides of metals of Groups III-VI of the Periodic Table of the Elements, by vapor phase reaction of vaporous metal halide and a vaporous source of boron, carbon, or nitrogen in a reactor at high temperatures, and relates particularly to the separation of entrained finely divided product from the reactor effluent stream.
U.S. Pat. No. 3,979,500 describes the preparation of Groups III-VI metal borides, carbides, and nitrides by reaction in the vapor phase of the corresponding vaporous metal halide with a source of boron, carbon, or nitrogen respectively in a reactor. U.S. Pat. No. 3,661,523 describes the preparation of finely divided titanium carbide from titanium tetrachloride in a vapor phase reaction, and states that product of improved purity may be obtained by heat treating the product, for example, by collecting the product at temperatures of from about 350.degree. C. to 1500.degree. C., in the substantial absence of air. The collection of product at such high temperatures reduces the level of adsorbed impurity, e.g., unreacted titanium tetrachloride. Air is excluded because the titanium tetrachloride would react with the oxygen or moisture in air upon contact to form titanium oxide coatings on the carbide power, rendering the powder less useful in the preparation of cemented carbide articles.
According to the patent, the titanium carbide can be collected and heat treated in one step, i.e., in the collection vessel, or the process can be performed in separate steps of recovery and heat treatment, which may but need not be successive. In a preferred embodiment, the reactor and the recovery equipment are maintained at 350.degree. C. or above, and the product is held in the receiver at collection temperatures of at least 350.degree. C. for a period of time sufficient to yield a product of reduced impurity, usually for a time sufficient to remove substantially all of the volatile halogen-containing species adsorbed on the product surface. Typical times are said to be from about 30 minutes to about 8 hours. The patent says that product can be held in a heated receiver or treated in a furnace or rotary calciner for, typically, at least 30 minutes.
For separation of product from the effluent gas stream, the patent describes the use of one or more cyclones which discharge product into receivers that optionally are purged with inert gas such as argon and are heated to maintain collected product at 350.degree. C. or higher. However, the use of the described recovery means has disadvantages. Thus, the product, which contains particles in a range of sizes from 0.02 to 1 micron, tends to become partly classified by size in the cyclone, so that the larger particles of product go into the receiver and the smaller particles are carried from the cyclone in the gas stream. If two or more cyclones are used in series, as shown in the FIGURE of the patent, even more classification will occur. In addition, a final filter such as the bag filter shown in the FIGURE, is needed to remove fine particles which escape the cyclone before the effluent gas stream, i.e., the waste gas, is vented. The fines trapped in the bag filter are wasted unless further steps are taken to recover and purify them and blend them with the product in the receiver. Because bag filters usually cannot be operated at the high collection temperatures desired, i.e., 350.degree. C. or higher, any product recovered from the bag filter would require separate treatment. The use of a scrub tower to remove fines from the effluent stream would entail further loss of product, and moisture-sensitive product would be contaminated in the tower.
German Offenlegungsschrift No. 2,523,423, relating to the preparation of submicron titanium boride powder, depicts a similar product recovery system employing cyclones and a final bag filter. The receivers are said to be heated to above 93.degree. C., e.g., from about 93.degree.-316.degree. C., to assist in degassing the product of impurities such as adsorbed unreacted reactants. Purging the receiver with inert gas is also mentioned. In a further purification step, the titanium diboride powder may be heated at 400.degree. to 1000.degree. C. for 1 to 4 hours in a furnace or rotating calciner.